SAMPLE: OPTARG: Optional Parameters with Automation

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Optional parameters are implemented by marking the parameter with the optional attribute and by using VARIANT as the type of the parameter. If the controller omits a parameter, the method will be passed a VARIANT with VARIANT.vt = VT_ERROR and VARIANT.scode = DISP_E_PARAMNOTFOUND.

Variable parameters are implemented by marking the method with the vararg attribute and by providing a parameter of type SAFEARRAY(variant). This array will contain the variable number of parameters passed by the controller.

NOTE: vararg methods do not have the upper limit of parameters that methods with optional parameters do.

Because of a problem in the OLE dlls, invoking the Multiply method will cause an exception in the 32-bit OLE that shipped with Windows NT 3.5 and a GP fault in 16-bit OLE 2.02 and 2.03. This problem has been fixed in NT 3.51 and Windows 95.

The problem occurs if a vararg method has a retval parameter. For example, the following will fail:

        [vararg]
        HRESULT Multiply([in] short RequiredInteger,
                      [in] SAFEARRAY(variant) VariableNumberOfIntegers,
                      [out, retval] long* retval);
				

The problem does not occur if the vararg method doesn't have a retval parameter. For example, the following will currently work:

        [vararg]
        HRESULT Multiply([in] short RequiredInteger,
                      [in] SAFEARRAY(variant) VariableNumberOfIntegers);
				

        [vararg]
       long Multiply([in] short RequiredInteger,
                      [in] SAFEARRAY(variant) VariableNumberOfIntegers);